This is part of a larger endeavor whose aim is to systematically investigate executive control functions, studying their development, neural bases, and genetic &neurochemical modulation from infancy through old age. The focus here is on inhibition of attention, inhibition of action, and cognitive flexibility during early development. It is hypothesized that: (1) Errors made by children of 3-10 years reflect problems with flexibly switching. Even 3-year-olds will succeed at steady-state attentional inhibition (selective attention) and steady-state action inhibition (inhibition of old stimulus-response mappings). Lags of up to 3-6 years will be found, however, between when children can first show inhibition in steady-state and when they can switch back and forth. (2) This progression does not depend on improved memory;even the younger children will be able to state the correct rule on each trial. It does depend, though, on improvements in the ability to execute the mental computations necessary to translate abstract rules into practice. The core problems in the development of executive control functions are the abilities to flexibly switch mental settings and to flexibly manipulate information in one's mind, not inhibition or memory per se. (3) Many of the errors made by infants and preschoolers reflect difficulty in grasping that two things are conceptually connected if they are not physically connected (e.g., not part of the same entity), and the flip side, difficulty grasping that attributes of a single entity can be separated and the entity conceptually redescribed from different perspectives. Thus, while switching attentional focus and response mappings is difficult, when the stimulus dimensions are separated, children will succeed at such switching at a very young age. The proposed research will test these and other hypotheses, charting the development of inhibitory control and cognitive flexibility in the early years of life. This will provide valuable insights into why children have difficulties, conditions for optimizing their performance, and benchmarks by which to assess children's development or deficits. A more refined understanding of executive control functions will assist efforts to understand how these are differentially affected in disorders such as Attention Deficit Hyperactivity Disorder, Obsessive Compulsive Disorder, schizophrenia, addictions, and autism. Insights into the conditions under which children can succeed may yield procedures that might prove helpful to those afflicted with disorders affecting these critical functions.